The present invention relates to devices and methods for sensing condensation conditions and for preventing or removing such condensation from surfaces such as vehicle windscreens, eyewear, goggles, helmet visors, computer monitor screen, windows, electronic equipment, etc., and especially devices and methods that use a thermal sensor and a humidity sensor in an adjacent ambient space with respect to the surface, or in thermally conductive contact with a thermoelectric cooler (TEC), for automatically and dynamically sensing condensation conditions when condensation appears on a surface or before such condensation actually appears on a surface.
The level of moisture in air at any time is commonly referred to as relative humidity. Percent relative humidity is the ratio of the actual partial pressure of steam in the air to the saturation pressure of steam at the same temperature. If the actual partial pressure of steam in the air equals the saturation pressure at any given temperature, the relative humidity is 100 percent. If the actual partial pressure is half that of the saturation pressure, the relative humidity is 50 percent, and so forth.
Dew point temperature, also known as condensation temperature or saturation temperature, is a function of the level of moisture or steam that is present in the air, and is the temperature at which air has a relative humidity of 100 percent. Condensation of moisture on a surface occurs when the temperature of that surface is at or below the dew point temperature of air surrounding the surface.
When air having a relatively high content of moisture comes into contact with a surface having a temperature at or below the dew point temperature, steam will begin to condense out of the air and deposit as water droplets onto the surface. At this time, a thin layer of liquid water comprised of small water droplets forms on the surface, creating a visual hindrance or xe2x80x9cfogxe2x80x9d to an observer looking at or through the surface. Once, formed, the condensation can be dispersed and removed either by raising the temperature of the surface, thereby changing the water into steam, or by lowering the relative humidity of the air surrounding the surface, thereby allowing the droplets to evaporate.
Steam, as a gas, exists in a saturated state at pressures and corresponding temperatures that are predictable and measurable. Notably, the standard for steam""s thermodynamic properties, including saturation pressures and temperatures, in the United States and arguably the world, is the ASME (American Society of Mechanical Engineers) Steam Tables. These thermodynamic property tables are readily obtainable from ASME, as well as from engineering texts.
In that steam possesses certain characteristics and traits as a saturated gas that are measurable and exact, equations have been developed that permit the engineer to approximate and predict the properties of steam at a desired set of conditions when its properties are known at a different, or datum, set of conditions. Such an equation, in the case of gas saturation pressures and temperatures, is entitled the Clausius-Clapeyron Equation. This equation, which may be described in several variations, may be best stated for the purposes at hand in the following form:       ln    ⁢          xe2x80x83        [                  P        2                              xe2x80x83                    ⁢          sat                            P        1                              xe2x80x83                    ⁢          sat                      ]    =                    Δ        ⁢                  xe2x80x83                ⁢        H            R        *          (                        1                      T            1                          -                  1                      T            2                              )      
where
P1sat is the saturation partial pressure at state 1, in units of psia;
P2sat is the saturation partial pressure at state 2, in units of psia;
xcex94H is the heat of vaporization, equal to approximately 755,087.46 (ftxe2x88x92lbf)/lbm for steam;
R is the gas constant, equal to approximately 85.8 (ftxe2x88x92lbf)/(lbmxe2x88x92xc2x0 R) for steam;
T1 is the temperature at state 1, in units of degrees Rankine; and
T2 is the temperature at state 2, in units of degrees Rankine.
Thus, using the Clausius-Clapeyron Equation, once steam ""s saturation pressure and temperature are known (the saturation pressure and temperature defining state 1 of the steam), given any other desired temperature, the saturation pressure at this temperature can be calculated to a high degree of accuracy (the temperature and calculated saturation pressure defining state 2 of the steam). Conversely, given any known state 1 conditions, for any desired saturated gas pressure, the saturation temperature can be calculated (the saturation pressure and calculated temperature defining state 2 of the steam).
The invention provides a device and method for sensing or predicting when condensation is present or imminent and for suppressing such condensation from a surface by preventing it or removing it. A first thermal sensor is in thermally conductive contact with the surface. A second thermal sensor is in an environment separated from the surface. A humidity sensor is in the environment of the second thermal sensor. A circuit causes a condensation suppression mechanism to be activated for preventing or removing condensation having the given physical state from the surface when a temperature sensed by the first thermal sensor, a temperature sensed by the second thermal sensor, and a humidity sensed by the humidity sensor indicate that a condensation condition is either present or likely and requires prevention or removal at the surface. As used herein and in the claims, the term xe2x80x9csuppressxe2x80x9d encompasses prevention or preclusion of condensation conditions as well as, in the alternative, removal of existing condensation conditions.
The invention provides a convenient and practical mechanism for detecting condensation conditions quickly, before they manifest themselves on the surface. In certain embodiments the condensation suppression mechanism can be activated automatically when a condensation condition is detected, thereby providing convenience and safety where the surface is a windscreen of a vehicle, for example, or goggles, a helmet visor, computer monitor screen, window, electronic equipment enclosure.
In one embodiment of the invention, the second thermal sensor is in thermally conductive contact with a cooling device, and a circuit activates the cooling device in order to maintain the second thermal sensor at a temperature that is lower than a temperature of the first thermal sensor. The humidity sensor is in thermally conductive contact with the cooling device. The circuit causes the condensation suppression mechanism to be activated when the humidity sensor indicates a presence of high humidity conditions or condensation at the temperature that is lower than the temperature of the first thermal sensor.
In alternative embodiments of the invention, the environment of the second thermal sensor is in an adjacent ambient space with respect to the surface. The circuit determines that the condensation condition requires suppression at the surface by determining, from the temperature sensed by the second thermal sensor and the humidity sensed by the humidity sensor, the pressure of steam in the environment of the second thermal sensor. Then, the circuit may either determine a ratio of the pressure of steam in the environment of the second thermal sensor to the saturated steam pressure at the temperature sensed by the first thermal sensor, or determine a difference between a temperature sensed by the first thermal sensor and a dew point temperature associated with the pressure of steam in the environment of the second thermal sensor.
Thus, in certain embodiments of the invention, instead of measuring RH at an intentionally lowered temperature relative to the surface in question, RH (and temperature) can be measured in the surrounding ambient air adjacent to and in the proximity of the surface itself. Through calculation, the measurements taken in the surrounding ambient air can be extrapolated using the Clausius-Clapeyron Equation or any of its derivatives to determine whether condensation conditions exist on the surface in question or are imminent. Thus, it is not necessary physically to create a simulated (state 2) temperature in which a (state 2) relative humidity (RH) value can be measured.
Numerous additional features, objects, and advantages of the invention will become apparent from the following detailed description, drawings, and claims.